Aluminum honeycomb panels are conventionally fabricated by the use of a plurality of strips of aluminum foil which are bonded to one another by the use of various types of synthetic resin adhesives. Conventionally, the adhesives are applied in parallel bands to the foil strips or sheets which are then successively assembled in juxtaposition to one another so that, when external tensional forces are applied, the stack expands to form a honeycomb core structure.
A detrimental aspect of the utilization of adhesively bonded honeycomb core in the fabrication of structural panels lies in the fact that the top and bottom surface sheets of said panels are frequently assembled in operative relationship with the honeycomb core by the use of resin adhesive materials which must be subjected to temperatures similar to those used in the fabrication of the honeycomb core structure in order to achieve the securement of the sheets to the honeycomb core which is interposed therebetween.
When the adhesively bonded honeycomb core is subjected to these elevated temperatures, the bond is substantially destroyed and the surface sheets are, consequently, maintained in operative relationship with each other by a loosely associated assemblage of corrugated aluminum foil strips.
The strips serve to maintain the surface sheets in operative relationship with each other when the sheets are subjected to loads in tension or compression, but the sheets can only resist shear loads longitudinally of the foil sheet or strips and, consequently, the panels assembled in the above-described manner are not utilizable in many critical structural applications.
Attempts have been made to utilize other means of securing the aluminum foil strips to one another to provide a honeycomb core, such as resistance welding or the like, but such attempts have proven fruitless because of the essential metallurgical structure of aluminum and, particularly, aluminum foils, which is not conducive to the weldment of said foils.
Stainless steel foil honeycomb core is widely used in structural panels and in jet engine seals, the corrugated foil strips constituting said core being welded at the apices or nodes of the corrugated foil strips or sheets to secure them in operative relationship with one another. Such stainless steel welded honeycomb core can be readily brazed to stainless steel sheets since the weldments resist the elevated temperatures which cannot be sustained by adhesively bonded aluminum honeycomb core.
An excellent example of such welded stainless steel honeycomb core is disclosed in U.S. Pat. No. 2,910,153 wherein the stainless steel strips constituting the core structure are welded to one another to provide a rigid, strong and thermally resistant structural panel which can be subjected to high mechanical loads and high temperatures without failure of the weldments and consequent failure of the panel in tension, compression and shear.
However, the manufacture of stainless steel honeycomb core fabricated from sheets or strips of stainless steel foil entails the use of resistance welding circuits and complicated machinery. In many instances, the weldment of the stainless steel sheets or strips to one another entails such high current usage that the panel which is welded to said strips must be subjected to a cooling bath by water or the like.
In addition, the welding process is characterized by the formation of relatively large weld nuggets which frequently constitute stress concentration areas subject to failure when the panels incorporating the stainless steel honeycomb core are subjected to high loads.
Again, titanium foils have been assembled into honeycomb core structures but, as disclosed in U.S. Pat. No. 3,598,953, the assemblage of titanium foil core strips in operative engagement with one another to constitute a core structure entails the utilization of high temperatures sufficient to result in diffusion bonding of the core strips to one another.
All of the prior art methods of fabricating honeycomb core structures have been characterized either by structural deficiencies resulting from the means of attachment of the core strips to one another, as in the case of aluminum or with stainless or titanium, by the necessity for expensive and time-consuming manufacturing processes.